The present invention relates to a disk array apparatus and a disk array apparatus control method.
In recent years, information processing systems operated for 24 hours without halt, such as commodity sale systems using the Internet, have increased. Data used in such an information processing system are often stored in a database on a disk array apparatus as the data quantity increases and improvement of fault tolerance is requested. In information processing systems operated without halt, it is necessary to back up data stored in the database without stopping database update processing. Therefore, a method of providing a replica volume for backup in a disk array apparatus and storing data stored in the database in the replica volume for backup as well is used. It is possible to acquire a backup of the database at a certain point in time by stopping writing data into the replica volume and copying the data stored in the replica volume into an external storage medium such as magnetic tape.
U.S. Pat. No. 6,141,773 discloses resynchronization processing of writing data written into a database while writing data into a replica volume is stopped, into the replica volume after completion of backup.
While the resynchronization processing is being executed, contents of the replica volume are not ensured. In some cases, therefore, two replica volumes are provided for one database, and the two replica volumes alternately conduct the resynchronization processing. If in this case a hardware fault occurs in a data area for storing the database, the database can be recovered by using one of the replica volumes and a REDO log of the database.
Besides the hardware faults, in the database operation, illegal data is written into the database in some cases because of malfunction in software, a user's operation mistake, or the like. Such a fault is called software fault. If a software fault occurs, it is necessary to restore data at a certain point in time preserved on magnetic tape or the like, and recover the database by using the restored data and REDO log. Since a considerably long time is required to restore data from the magnetic tape or the like, the halt time of the system becomes long.
When a software fault has occurred, therefore, it is demanded to recover the database quickly by using data in the disk array apparatus without restoring data from the external storage medium such as magnetic tape. In the case where the above-described two replica volumes are used, a storage capacity which is three times that of the volume storing the database becomes necessary, and the cost required to introduce a disk array apparatus increases.
Therefore, it is demanded to reduce the storage capacity required to back up the database against a hardware fault or a software fault.
Furthermore, in the resynchronization processing, it is necessary to read out data to be written into the replica volume from the database. This results in a problem that the performance of external access to the database is degraded during the resynchronization processing. Therefore, it is demanded to quickly recover the database without lowering the performance of external access to the database.
Furthermore, apart from the quick recovery of the database, it is demanded to acquire a backup at a certain point in time to provide against an emergency, without stopping the database update processing.